= Resonance spin statistical factor
{title2=$g=(2J_R+1)/[(2J_a+1)(2J_b+1)]$}
For unpolarized entrance particles, average over their $(2J_a+1)(2J_b+1)$ <spin> orientations and sum over the isolated resonance's $2J_R+1$ magnetic substates. The resulting weight multiplies the partial-wave <cross section>. It can exceed one because orbital <angular momentum> participates; it is not a standalone probability of forming a state. Polarization or several unresolved resonance <spins> requires more detailed channel counting.
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